As apparent, for example, from Unexamined Japanese Patent Publication No. 88281/1983, a method is already well known of lining a pipe by forming a lining from a rigid or semi-rigid plastic tube enlarged within the pipe.
When such a known method of lining a pipe with a plastic tube inserted in the pipe by enlarging the tube is so adapted that the tube is enlarged progressively from the starting end toward the terminal end, the air present between the inner surface of the pipe and the outer peripheral surface of the plastic tube is properly discharged toward the terminal end with the progress of enlargement of the plastic tube. The method then gives a lining of high quality without trapping air.
The known lining method employs a fluid of high temperature as means for applying heat and pressure to enlarge the plastic tube. However, if the high-temperature fluid is supplied directly to the interior of the plastic tube, heat and pressure are applied to the entire length of the tube substantially at the same time to enlarge the tube. Air is then liable to remain between the pipe and the lining, making it impossible to form a lining of high quality.
In this case, the plastic tube can be enlarged progressively by providing a pig inside the plastic tube and applying heat and pressure to the plastic tube with the movement of the pig through the tube, with the high-temperature fluid supplied to the interior thereof as disclosed in the foregoing prior-art publication. Nevertheless, the high-temperature fluid, if used in combination with the pig, is unable to heat the portion of the plastic tube toward the direction of advance of the pig, with the result that the pig fails to move through the tube smoothly and inevitably slows down to make the lining operation inefficient. Most suitably, the high-temperature fluid is steam since steam has a great heat capacity and condenses to pure water which does not contaminate the lining, but the use of steam at an increased internal pressure to give a high temperature renders the operation hazardous. In view of the safety of the operation, therefore, the internal pressure of the tube is limited to about 2 to about 3 kg/cm.sup.2, whereas the highest temperature available at this pressure level is only about 120.degree. to about 130.degree. C. Thus, steam is not usable for plastic tubes of relatively high softening point, such as polytetrafluoroethylene tubes. Moreover, there arises a need to install a boiler and other equipment attached thereto, hence an increased equipment cost.